My limited experience with mirrored features was very frustrating. I found a number of legacy models that were heavily mirrored, apparently generated in some version prior to SW2007, that 'broke' in illogical ways with strange error messages when manipulated, modified, or just inserted in assemblies with SW2010. Apparently the older versions of SW were 'better' in some way associated with mirroring. I gave up and recreated what I needed from scratch, with relatively little mirroring.
Mike Halloran
Pembroke Pines, FL, USA